1. Field of the Invention
The present invention relates to load cells for measuring static and slowly fluctuating load. More particularly, the present invention relates to a variable reluctance load cell for measuring the load, or tension, on static devices in an environmentally hostile environment (e.g., determining mooring line tensions of offshore oil platforms).
2. Description of the Related Art
Offshore deepwater platforms utilize various mooring systems for stationkeeping. A number of offshore platforms e.g. spars, deep draft caissons, semisubmersibles and floating production, storage and offloading vessels (FPSOs) are equipped with a means to jack the mooring chain and maintain tension on the line, reducing the amount of slack. Passively moored platforms or vessels that are not required to move may not be equipped with a permanently mounted tensioning system. As the mooring lines experience fatigue and stretch, the platform can twist, leading to increased friction between the links, and accelerating fatigue and failure. The amount of tension on the mooring line determines the amount of slack, and consequently the amount of relative movement of the platform or vessel.
Fairleads are used to attach the mooring chain to the deck of the platform or vessel. In one configuration, a chain stopper latches the chain outboard of the fairlead and allows the stopper to rotate freely about two perpendicular axis. All motion change between the mooring line and the vessel occurs on proper bearing surfaces, and not between the fairlead and the chain.
Tension in the vertical chain leg between the fairlead and the deck level stopper, combined with the rotation of the fairlead caused by yawing of the platform or vessel, promote undesirable wear in the chain links.
Similarly, suspension bridges rely on large cables to maintain support for the bridge span. The amount of tension on the suspension cables is indicative of the stress placed on the cables, and the amount and rate of cable wear and or fatigue.
Various devices are available to measure the amount of tension, or applied tensile force, placed on fasteners and securing lines, including strain gauge bridges, differential transformers, capacitance sensors and variable reluctance load cells.
Mooring line tensions have been measured with instrumented chain links that employ strain gauged shear pins, strain gauges and strain gauge load cells. Generally, these devices have not been reliable for long term applications in hostile environments, for example, marine and aerospace environments. Strain gauges require adhesive attachment to the surface being measured. In non-controllable environments, strain gauges are subject to drift caused, for example, by adhesive breakdown, requiring recalibration. In environmentally hostile environments, the frequency of recalibration, repair or replacements becomes expensive, and may even be dangerous to perform.
Variable reluctance load cells use a variable reluctance transducer to measure force. For example, a core and winding can be used to sense changes in proximity to a cantilevered spring. Changes in inductance, caused by changes in the gap between the core and the spring, are reflected in the frequency of an oscillator circuit. In a previous load cell sensor utilizing a variable reluctance transducer, the load cell sensor responded primarily to forces along a pre-selected axis, while being relatively insensitive to both forces along axis transverse to the pre-selected axis, and to bending moments. Accordingly, the load cell sensors were mounted in the middle of the structure under observation to compensate for bending forces. Additionally, the load cell sensor was internal to a load-measuring unit, and contained all the components. Intrusion of contaminants into the sensor region could lead to premature aging of the components, including corrosion, making the readings unreliable.
Accordingly, there is a need for a load measurement device that is less prone to the various effects of exposure to hostile environments and can take into account effects of bending.
This invention provides a variable reluctance sensor for measuring the load, or tension, on static devices in an environmentally hostile environment.
A sensor in accordance with the invention uses opposing magnetic cores contained in a support tube. Each of the magnetic cores is attached to opposing ends of the support tube. Thus, as the support tube expands along the tube axis, the ends of the support tube, which are perpendicular to the tube axis, separate. A magnetic circuit is formed having an inductance defined by the size of the gap between the magnetic cores. Accordingly, when the magnetic cores attached to the tube ends separate, the size of the gap between the magnetic cores is increased. Thus, when the inductance is altered, the amount of expansion that has occurred can be determined. Knowing the elastic characteristics of the support tube material, the amount of force applied to the support tube can be calculated. Similarly, contraction of the support tube results in a change in inductance that is indicative of the amount of stress reduction. Alternatively the support tube can have very little stiffness relative to the structure that it is mounted on so that no load passes through the support tube and it merely displaces the same amount as the structure displaces in the region between the attachment points. The combination is tested under known loads to provide the calibration.
Preferably, one of the magnetic cores is generally C-shaped, and attached to an end plate by way of a bracket. The end plate may be one of the tube ends, or another plate that is in turn attached to the support tube. The C-shape is preferred for one of the magnetic cores so that the windings can be placed at the ends of the C-shaped gaps. The other magnetic core is preferably I-shaped, and is attached to a second end plate by way of a second bracket. The second end plate, like the first end plate, may be the other tube end, or another plate that is in turn attached to the support tube. Thus, a cavity within the support tube containing the sensor is formed. Preferably, the cavity containing the sensor is sealed in a manner to prevent water or other damaging agents from entering the cavity and damaging the sensor or its wiring. The cavity can also be filled with a low durometer elastomeric potting material, silicon oil, or any other suitable material for protection of the components from environmental agents such as water. The choice of the elastomeric potting material can be selected according to the anticipated environmental exposure of the sensor. For example, a low out-gassing material may be appropriate if the sensor is used at high altitude or space while a low compression material may be better if the sensor is used below sea level, such as underwater or underground.
An excitation coil is wound around the poles on one of the magnetic cores, and provides electrical connection for an inductance whose value is variable as a function of the widths of the gaps, and also the axial distortion of the support tube. In the preferred embodiment, there are two excitation coils, each surrounding a separate end of the C-shaped core. This arrangement minimizes non-linearity of response due to fringing effects. The wires from the two coils are twisted and attached to cabling that connects them to external circuitry. Thus, when excited by an external AC voltage, the C-core, the I-core and the gap between the C and I cores form an element of a magnetic circuit. The reluctance of this element is dominated by the gap because the C and I cores are fabricated from high permeability magnetic materials having very little reluctance. The sensor inductance is coupled with a fixed, predetermined capacitance in a resonant inductance-capacitance (LC) circuit. The resonant frequency of the LC circuit is a function of the gap between the C-shaped and I-shaped cores. Accordingly, changes in the gap dimension results in a change in oscillation frequency. Since the only active component in the sensor is the number of excitation coils, the sensor is immune to drift.
To measure the load on a static device, for example, a chain that moors a marine platform, the support tube is fixedly attached to the surface of a sensor link, and the sensor link placed as a link in the chain. The support tube can be attached to the surface of the sensor link using bolts, by welding, or any other suitable attaching means. In order for the sensor to measure the load on the sensor link, it is preferred that the sensor tube material and the sensor link material are compatible, more preferably the same material or material having the same or similar coefficient of thermal expansion. In the preferred embodiment, the support tube and the sensor link are made of steel. When used in marine applications, it is preferred that a protective coating is applied to the support tube and the sensor link.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.